Which of the following complexes give white ppt when treated with aqueous solution of \(BaCl_2\)?
A. \([Co(NH_3)_5SO_4]Cl\)
B. \([Co(NH_3)_5Cl]SO_4\)
C. \([Co(NH_3)_5(NO_2)](NO_2)_2\)
D. \([Co(NH_3)_5CN]SO_4\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → B, D only
\( \ce{BaCl2} \) gives a white precipitate of \( \ce{BaSO4} \) only with free SO42− ions, i.e. when sulphate is the counter ion outside the square bracket:
\( \displaystyle \ce{Ba^2+ + SO4^2- -> BaSO4 v} \)
A. \( \ce{[Co(NH3)5SO4]Cl} \) — No precipitate. \( \ce{SO4^2-} \) is a ligand inside the coordination sphere; only \( \ce{Cl-} \) ionises.
B. \( \ce{[Co(NH3)5Cl]SO4} \) — Gives a white precipitate. Sulphate is the counter ion and ionises.
C. \( \ce{[Co(NH3)5(NO2)](NO2)2} \) — No precipitate. It contains no sulphate at all.
D. \( \ce{[Co(NH3)5CN]SO4} \) — Gives a white precipitate. Sulphate is the counter ion and ionises.
A and B are ionisation isomers: they differ in which anion is the ligand and which is the counter ion.
NCERT (2026-27 edition):
Ionisation Isomerism – “This form of isomerism arises when the counter ion in a complex salt is itself a potential ligand and can displace a ligand which can then become the counter ion. An example is provided by the ionisation isomers [Co(NH3)5(SO4)]Br and [Co(NH3)5Br]SO4.”